The present invention relates in general to an electrochromic display and, more particularly, to a driving circuit for an electrochromic display having a plurality of segment electrodes to be colored, the driving circuit being provided for uniformizing the depth of coloration and for insuring a uniform bleaching condition of the segment electrodes.
It is preferable that an electrochromic display be driven by the constant current driving method in a coloring mode and by the constant voltage driving method in an erasing mode, as disclosed in a copending Patent Application Ser. No. 915,003 filed June 13, 1978, assigned to the present assignee, for example.
The above stated driving method disclosed in Patent Application Ser. No. 915,003 had disadvantages that if and when a driving circuit was incorporated into a semiconductor chip, variations in the electrical properties of driving elements such as transistors formed within the semiconductor chip led to non-uniformity of coloration degree or depth among a plurality of segment electrodes included within an electrochromic display. The variations come from changes of conditions in manufacturing the driving elements.
In order to overcome these and other problems there are proposed in assignee's U.S. Patent Application Ser. No. 056,629 by H. Fukuda et al, filed July 11, 1979, an improved driving circuit for coloring all the segment electrodes included within an electrochromic display in a uniform coloration.
However, there were unsolvable problems in the driving circuit as disclosed in U.S. Patent Application Ser. No. 056,629 in that an appreciable degree in non-uniformity of coloration depth among the segment electrodes existed because of reasons present both in the driving circuit integrated and in cell structure of the electrochromic display cell.
The reasons for the above were that it was difficult to accurately control the modification of the size of the transistors enough to allow for the variations in the size of the segment electrodes because of the difficulty in controlling the process therefor. The variations in electronic properties of the transistors led directly to that of the constant current value.
The same was true of the fact that the size of the segment electrodes deviated from intended calculation values because of variations in their manufacturing conditions, for example, etching procedures. Especially, the variations in the size of some small segment electrodes became increasingly critical.
Therefore, it is desired that the change in coloration depth in all of the segment electrodes be completely eliminated with respect to the constant current driving circuit in the coloring mode and the constant voltage driving circuit in the erasing mode.